DOC: internal: document the init calls

INITCALLs are used a lot in the code now and were not documented, resulting
in each user having to grep for functions in other files. This doc is not
perfect but aims at improving the situation. It documents what's been
available since 1.9 and may be backported there if it helps though it's
unlikely to be needed as it's mostly aimed at developers.
1 file changed